A time-to-digital converter (TDC) can be used for a variety of purposes. For example, a TDC can be used to measure the duration of time that has elapsed between a START and a STOP pulse or any other timing event. It can also be used to output the time of arrival for an incoming pulse. High resolution TDCs are increasingly popular in many applications, including time of flight measurements, phase detectors in phase-locked-loops (PLLs), data converters, high speed signal capturing, demodulators, and other measurement or instrumentation applications.
Conventional TDC systems allow a single TDC to perform only single measurements at any one time. This means that simultaneous measurements of 2 or more time intervals cannot be performed by a single TDC. Multiple TDCs have to be provided to measure multiple time intervals simultaneously. This increases the area consumption of the TDC system. Further, the TDC cannot be started immediately after the termination of the previous measurement. Not only does this limit the type of measurements that can be performed, it also slows down the operating efficiency of the TDC system. Dead or inert time slots, during which no acquisition can be performed, have to be injected when the TDC is calibrated. Calibration is essential, especially for high-precision applications, because process variations and component deviations can cause undesirable offsets in time and gain errors in the TDC converter characteristics. In conventional systems, calibration is typically performed in fixed calibration intervals, which is undesirable because the TDC is unable to respond quickly to changes. Moreover, some systems do not allow interruptions in operation for calibration purposes.